The ultimate nightmare for any comedian is that a heckler will raise more laughs. Any comedian worth his or her salt will therefore have as part of his/her comedicarmoury a series of devastating put-downs. The following is the best I've ever heard (unfortunately I don't know who the comedian was):

The Lucid Heckle:
Occasionally a comedian will say something that just begs for someone to respond to with wit. Sometimes someone does. One of the best examples I've ever witnessed was in a comedy club in Cork. The comedian was from England and asked the audience if anyone spoke a foreign language. Quick as a flash, someone yelled out, "yes, English. You bastard."

But possibly the best ever was the apocryphal story of Kirk Douglas's son. Apparently he tried his hand at comedy for a while, and was awful. The gig wore on and the audience started rounding on him. He got sick of it and yelled out, "dammit, don't you know who I am? I'm Kirk Douglas's son."

The Come-And-Get-Me Heckle:
Occasionally a comedian will say something that just begs for someone to respond to with wit. Sometimes someone tries and fails, but in doing creates a perfect opening for the comedian . One such heckle occurred at last year's Edinburgh festival. A comedian was getting tons of abuse from a guy down the front. He got fed up with him and said, "listen mate, at least I'm doing something. When's the last time you made a contribution to the human race."

The heckler responded "actually I spent today in the park with my dying anorexic sister."

The audience took a sharp intake of breath. The comedian though didn't blink. He calmly replied, "really? I bet that was no picnic."

The Conversational Heckle:
If you're heckling, heckle once and let it go. Some people don't realise this and will continue speaking to the comedian, until said perform is forced to order them to shut the fuck up. I witnessed a pretty odd example of this, when a very strange, drunk guy insisted on responding to every single line uttered by every comedian. Eventually one person said, "look, if you're so talkative, why don't you just come up here and try it." Immediately the heckler jumped to his feet, walked onto the stage, pulled out a huge wad of A4 paper and shouted, "I'd like to read you all a short story of mine." It took two bouncers to remove him.

The Not Very Funny Heckle:
The worst kind. Closely related to the lucid heckle, with one basic flaw: it's not funny. It's a horrible thing to witness. Often the comedian will pause for a few seconds to try and think of a comeback, before realising that nothing could possibly be funny. The heckler feels like an idiot. The audience gets bored.